1 2 . G E N E T I C C Y T O L O G Y
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individuals with supernumerary X chromosomes, when the number of
X chromosomes increases, the number of sex chromatin units increases
accordingly, generally at the ratio of n : n—1 respectively, i.e. the number
of sex chromatin units in the interphase nucleus is one fewer than the
number of X chromosomes in metaphase. In X X X or X X X Y individuals, two pieces of sex chromatin can be seen in each nucleus, and in
X X X X or X X X X Y individuals, three. This fact also established the
connexion between the sex chromatin and the sex chromosome.
Other observational data also support the view that a single X is
responsible for the presence of the sex chromatin. In animal species with
heterogametic females (such as domestic chick), sex chromatin is also
found (Kosin and Ishizahi, 1959). But the strongest evidence for the single-X hypothesis has been the allocyclic behavior of one X chromosome
in somatic prophase of female cells. One question is still unanswered,
namely, whether the sex chromatin seen in interphase represents the
entire X or a part of it. Merely by size, it seems that the sex chromatin
is only a part of the X ; this again is circumstantial evidence. The
point has some important bearings in elucidating the behavior of the
sex-linked genes.
Not all mammalian species are the same in respect to the behavior
of the sex chromosomes. Autoradiographic data beautifully presented
by Taylor (1960a,b) show that both X chromosomes in somatic cells
of female Chinese hamsters have a delayed D N A synthesis period as
compared with the autosomes. Also the two X chromosomes behave
differently in respect to the labeled regions, one of which has an asynchronous D N A label along the entire chromosome and the other along
only the longer arm. This phenomenon led Yerganian and his associates
(1960) and Yerganian (1961) to propose an X X X 2 Y sex-determination
scheme for mammals. According to this hypothesis, the X x is a neutral
element, the X 2 is female determining, and the Y, male determining.
The constitutions for the female and male would be X X X 2 and X X Y
respectively. Yerganian, Kato, Leonard, Gagnon and Grodzins (1960)
also attempted to interpret the various sex anomalies in man by implying that the two X chromosomes in man are also intrinsically different.
The weakness of the above mentioned hypothesis is that it must
assume a selective fertilization among the female and male gametes.
That is to say, if fertilization is a random affair, the Xj-bearing ovum
and the X 2 -bearing ovum should have an equal chance to accept the
Xx-bearing or the Y-bearing spermatozoan. Thus the number of
X i X i females and X 2 Y males should be statistically the same as the
X t X 2 females and the X X Y males. Since this in not the case, and a
selective fertilization procedure is rather inconceivable in respect to
one chromosome, this theory is not at all attractive. An alternative is that
447
individuals with supernumerary X chromosomes, when the number of
X chromosomes increases, the number of sex chromatin units increases
accordingly, generally at the ratio of n : n—1 respectively, i.e. the number
of sex chromatin units in the interphase nucleus is one fewer than the
number of X chromosomes in metaphase. In X X X or X X X Y individuals, two pieces of sex chromatin can be seen in each nucleus, and in
X X X X or X X X X Y individuals, three. This fact also established the
connexion between the sex chromatin and the sex chromosome.
Other observational data also support the view that a single X is
responsible for the presence of the sex chromatin. In animal species with
heterogametic females (such as domestic chick), sex chromatin is also
found (Kosin and Ishizahi, 1959). But the strongest evidence for the single-X hypothesis has been the allocyclic behavior of one X chromosome
in somatic prophase of female cells. One question is still unanswered,
namely, whether the sex chromatin seen in interphase represents the
entire X or a part of it. Merely by size, it seems that the sex chromatin
is only a part of the X ; this again is circumstantial evidence. The
point has some important bearings in elucidating the behavior of the
sex-linked genes.
Not all mammalian species are the same in respect to the behavior
of the sex chromosomes. Autoradiographic data beautifully presented
by Taylor (1960a,b) show that both X chromosomes in somatic cells
of female Chinese hamsters have a delayed D N A synthesis period as
compared with the autosomes. Also the two X chromosomes behave
differently in respect to the labeled regions, one of which has an asynchronous D N A label along the entire chromosome and the other along
only the longer arm. This phenomenon led Yerganian and his associates
(1960) and Yerganian (1961) to propose an X X X 2 Y sex-determination
scheme for mammals. According to this hypothesis, the X x is a neutral
element, the X 2 is female determining, and the Y, male determining.
The constitutions for the female and male would be X X X 2 and X X Y
respectively. Yerganian, Kato, Leonard, Gagnon and Grodzins (1960)
also attempted to interpret the various sex anomalies in man by implying that the two X chromosomes in man are also intrinsically different.
The weakness of the above mentioned hypothesis is that it must
assume a selective fertilization among the female and male gametes.
That is to say, if fertilization is a random affair, the Xj-bearing ovum
and the X 2 -bearing ovum should have an equal chance to accept the
Xx-bearing or the Y-bearing spermatozoan. Thus the number of
X i X i females and X 2 Y males should be statistically the same as the
X t X 2 females and the X X Y males. Since this in not the case, and a
selective fertilization procedure is rather inconceivable in respect to
one chromosome, this theory is not at all attractive. An alternative is that